1. Field of the Invention
The present invention relates to a toner for the use in developing electrostatic latent images in an electrophotographic image forming process or the like, a two-component developer, a developing device, and an image forming apparatus.
2. Description of the Related Art
The toner for the use in electrophotography can be obtained by a kneading pulverization method or a polymerization method such as a suspension polymerization method and an emulsion-polymerization aggregation method. In the kneading pulverization method, a toner raw material containing a binder resin and a colorant as main ingredients, in to which additives such as a charge control agent and a release agent are mixed as required, is melt-kneaded, then cooled to be solidified, and further pulverized and classified. To such a toner, inorganic fine particles such as fine-particle silica or aluminum oxide are generally added as external additives for the purpose of improving flowability and electrostatic charge stability.
In recent years, enhancement of image quality in the electrophotographic field has been studied from various angles. And there is a growing recognition that reduction in size of toner matrix particles is especially effective for enhancement of image quality. However, as toner matrix particles advance in size reduction, there develops a tendency that their flowability and transfer efficiency are lowered to result in formation of poor-quality images.
Such a tendency is brought to the fore in the case of forming full-color images in particular. In general, toners of subtractive primary colors, namely three colors of yellow (Y), magenta (M) and cyan (C), or toners of four colors including these three colors and black (B) are used for color reproduction in full-color image formation. The color reproduction procedure is, e.g., as follows, Electrification, exposure, development and transfer steps in an image forming process are repeated for each of C, M, Y and B colors, and then toner images thus formed on photoreceptors assigned for those colors respectively undergo primary transfer onto an intermediate transfer body in succession so that one color image is superposed upon another color image. Thereafter, a full-color image is formed on a recording medium by one-operation transfer of superposed toner images of multiple colors. In the final fixing step, the superposed toner images are fused and fixed into the recording medium.
In such full-color electrophotography, flowability and transfer efficiency of toner of each color become extremely important factors because development is carried out for plural times and in the transfer step, toner images are collectively performed in one operation. Therefore, to a toner small in particle size used in full color image formation, an external additive is often added which is composed of so large particles as having a volume average particle size of the order of 50 nm to 150 nm, for the purpose of enhancing flowability and transfer efficiency of the toner.
However, the external additive of large particles added to surfaces of toner matrix particles of the toner moves to reentrants in the surfaces of toner matrix particles of the toner and is deposited thereon and further embedded therein by various mechanical stresses applied to the toner through stirring in a developing device. Therefore, there occurs a problem that initial toner properties including the flowability and the chargeability the toner has at the time of initial use cannot be retained over the long term to result in degradation of image quality, such as background fog and inconsistencies in density.
When the kneading pulverization method is adopted, the reentrants in surfaces of toner matrix particles, where deposition and embedment of an external additive occur, are formed by the toner matrix particles undergoing stresses inside a device used in the pulverizing step and collisions between themselves. When the polymerization method such as an emulsion-polymerization aggregation method is adopted, on the other hand, those reentrants are formed of cavities among aggregated particles. In these reentrants, large ones with diameters reaching several μm are included.
In order to solve the problem, arts of reducing the number of reentrants in surfaces of toner matrix particles and smoothing the surfaces have been proposed. For exampler, in the case of the nonmagnetic toner disclosed in Japanese Unexamined Patent Publications JP-A 11-295922 (1999) and JP-A 11-295925 (1999), momentary heating treatment is given to toner particles so that the value obtained by dividing the reduced particle size D determined from their BET specific surface area by the 50% relative particle size d50 of particle size-specific relative weight distribution becomes 0.20 or above. By this treatment, surface conditions of the toner particles improve in not only smoothness but also uniformity, surface properties of the toner particles are retained stably over the long term, and flowability and electrification properties required of the toner are obtained.
Although the momentary heating treatment is given to toner matrix particles with the intention of enhancing their surface smoothness in JP-A 11-295922 and JP-A 11-295925, there is a risk that the release agent incorporated in the toner bleeds from the toner surface by this momentary heating treatment and affects adversely durability of the toner. In addition, there is another risk that the fluidizing agent added is fixed to surfaces of toner matrix particles by such treatment, and pressed and embedded in the surfaces of toner matrix particles by stresses from, e.g., stirring inside the developing device, resulting in degradation in flowability. Furthermore, retention of a homogeneous dispersion state during the momentary heating treatment requires a greater amount of fluidizing agent than usual, and threatens to impair electrostatic charge stability.